1. Field of the Invention
The present invention relates to a color cathode ray tube to be used in a direct viewing type color TV receiver or a terminal color display and, more particularly, to a color cathode ray tube which has its resolution improved all over its screen area by improving the structure of a main lens for controlling the shape of an electron beam deflected to the peripheral portion of the screen.
2. Description of the Prior Art
In a color cathode ray tube, generally speaking, there are mounted in a vacuum enclosure made of glass or the like a fluorescent face formed of fluorescent films of fluorescent materials of three colors of red (R), green (G) and blue (B) colors, a shadow mask acting as electrodes for selecting color selecting electrodes elements, and an electron gun for emitting three electron beams, so that a predetermined color image is reproduced on the fluorescent face by modulating the aforementioned three electron beams with image signals of R, G and B colors.
FIG. 1 is a section for explaining the construction of a shadow mask type color cathode ray tube as the color cathode ray tube of this kind. Reference numeral 1 designates a panel portion; numeral 2 a neck portion; numeral 3 a funnel portion; numeral 4 a fluorescent film; numeral 5 a shadow mask; numeral 6 a mask frame; numeral 7 a magnetic shield; numeral 8 a shadow mask suspending mechanism; numeral 9 an in-line type electron gun; numeral 10 a deflection yoke; and numeral 11 an external magnetic device for centering and purity corrections.
In FIG. 1, the three electron beams (i.e., a central electron beam Bc and side electron beams Bs.times.2) emitted horizontally on one line (in-line) from the electron gun 9 are deflected by the horizontal and vertical magnetic fields, which are generated by the deflection yoke 10 mounted on the transitional region between the funnel portion 3 and the neck portion 2, and have their colors selected by the apertures of the shadow mask 5 until they impinge upon the predetermined fluorescent materials.
The shadow mask 5 is supported by the mask frame 6 and is suspended and held on the inner wall of the skirt portion of the panel portion through the suspending mechanism fixed on that mask frame.
On the mask frame 6, there is mounted the magnetic shield 7 which has a function to shield the electron beams from the external magnetic fields (e.g., the terrestrial magnetism) thereby to prevent the impinging positions of the electron beams from being displaced by the external magnetic fields.
In this color cathode ray tube, the resolution at the screen periphery is deteriorated due deflection defocusing caused by the self convergence deflection yoke. With the self convergence deflection yoke, the center and side beams can converge all over the screen. However, the yoke has the strong astigmatism that overfocuses the electron beams in the vertical cross section and extends the vertical spot size.
In order to reduce the deterioration of the resolution, the structure of the focusing lens system of the electron gun has been improved.
FIG. 2a is a schematic diagram, as taken in section along the tube axis, for explaining the construction of an electron gum according to the prior art for improving the resolution; FIG. 2b is a section as taken along line 101--101 of FIG. 2a; and FIG. 2c is a front elevation of an electrode plate. Reference numeral 21 designates a cathode; numeral 22 a G.sub.1 electrode; numeral 23 a G.sub.2 electrode; numeral 24 a focusing electrode; numeral 25 an accelerating electrode; and numeral 26 a shielding cup.
In these Figures, the cathode 21, the G.sub.1 electrode 22 and the G.sub.2 electrode 23 constitute an electron beam generating portion, from which the electron beams are emitted along the initial passages arranged generally in parallel with a horizontal plane until they impinge upon the main lens portion.
This main lens portion is constructed of the focusing electrode 24 acting as the main lens electrode, the accelerating electrode 25 and the shielding cup 26.
The focusing electrode 24 is divided into a first kind of focusing electrode 241 and a second kind of focusing electrode 242, the former of which is formed with a single horizontally elongated aperture and equipped therein with an electrode plate 245 having three circular electron beam passing holes.
On the other hand, the second kind of focusing electrode 242 is formed with three circular electron beam passing holes at the end face confronting the first kind of focusing electrode 241. To the second kind of focusing electrode 242, there are attached plate-shaped correcting electrodes 243 (as will also be shortly called the "plate electrodes") which are extended toward the first kind of focusing electrode 241 in parallel with the array direction of those electron beam passing holes.
The electron beam passing holes of the electrode plate 245 and the focusing electrode 242 are given common axes and diameters for the individual electron beams.
The plate-shaped correcting electrode and the electrode plate 245 have their electron beam passing holes confronting each other to form the electrostatic quadrupole lens.
Moreover, the first kind of focusing electrode 241 is supplied with a constant focusing voltage Vf at 5 to 10 kV, and the second kind of focusing electrode 242 is supplied with a dynamic voltage Vd in superposition over the constant focusing voltage Vf.
On the other hand, the accelerating electrode 25 is supplied with a final accelerating voltage at 20 to 35 kV.
The aforementioned dynamic voltage Vd has a waveform in which a parabolic waveform having a period of the horizontal deflection period 1H and a parabolic waveform having a period of the vertical deflection period 1V of the electron beams are synthesized.
When the electron beams are not deflected at the central portion of the screen, the dynamic voltage drops to 0 so that not only the potential difference between the first kind of focusing electrode 241 but also the second kind of focusing electrode 242 but also the electrostatic quadrupole lens action substantially disappear. When the electron beams are deflected toward the screen corner portions (i.e., the peripheral portions), on the other hand, the dynamic voltage is maximized to maximize not only the potential difference between the first kind of focusing electrode 241 and the second kind of focusing electrode 242 but also the electrostatic quadrupole lens action.
When the electron beams are thus deflected, the dynamic voltage Vd is raised according to the increase in the deflection. As this dynamic voltage Vd rises, the quadrupole lens to be formed in the confronting portion between the first kind focusing electrode 241 and the second kind of focusing electrode 242 is intensified to correct the astigmatism resulting from the electron beam deflection.
At the same time, the voltage difference between an accelerating voltage Eb of the accelerating electrode 25 and the voltage applied to the second kind of focusing electrode 242 can be reduced to elongate the distance between the main lens and the electron beam focal point to focus the electron beams even on the screen peripheral portion.
By employing such electron gun, the resolution of the screen peripheral portion of the color cathode ray tube is drastically improved.
Specifically, the astigmatism to horizontally extend the electron beams deflected to the screen periphery by the self-converging magnetic field is corrected by the astigmatism to vertically extend the electron beams by the electrostatic quadrupole lens. At the same time, the corrections are also made upon the field curvature aberrations.
This field curvature aberration is an aberration which will deteriorate the resolution because the focusing conditions go out of the optimum ones in the screen periphery when the electron beam is focused in optimum at the screen center due to the difference between the distance to the screen center and the distance to the screen periphery from the main lens.
The intensity of the main lens final stage lens to be formed between the accelerating electrode and the second kind of focusing electrode when the dynamic voltage is applied is reduced so that the deflected electron beams can be focused in optimum in the screen periphery to correct not only the astigmatism but also the field curvature aberration.
Incidentally, if the electron gun having that electrostatic quadrupole lens is used, the action (i.e., the so-called "STC: Static Convergence") to converge the three electron beams upon the screen by the main lens final stage lens fluctuates with the fluctuation of the dynamic voltage Vf, to raise a problem of the convergence misalignment.
In the electrode structure of the type described with reference to FIG. 2a, this problem of convergence misalignment is solved by fluctuating the STC in the opposite direction at the electrostatic quadrupole lens portion to mutually cancel the STC fluctuations at the main lens final stage lens.
In the color cathode ray tube using the electron gun of the aforementioned type, however, the following problems arise due to the electrode construction of the electron gun.
Specifically, in order to fluctuate the STC by the electrostatic quadrupole lens, the horizontal electric field is applied to only the side electron beams so that these side electron beams are horizontally moved.
FIG. 3 is a section of an electrostatic quadrupole lens portion of the electron gun shown in FIG. 2a for explaining the operations of the same.
In FIG. 3, the plate electrodes 243 are fitted in the first kind of focusing electrode 241 and connected with the second kind of focusing electrode. Reference numeral 201 designates equipotential lines indicating the potential distribution which is established in the section of the plate electrodes 243, and numerals 202, 203 and 204 designate the same electric fields.
The electric field 202 to be established in the sections of the plate electrodes 243 contains not only the horizontal component 203 but also a small quantity of the vertical component 204 to be established by the quadrupole lens effect, so that the electrostatic quadrupole lens is intensified against the side electron beams to cause an unbalance from the astigmatism correction sensitivity for the central electron beam.
As a result, if the dynamic voltage is set to such a proper value as to correct the astigmatism of the side electron beams in the screen periphery, the astigmatism cannot be corrected for the central electron beam. If, on the other hand, the dynamic voltage is set to a proper value for the central electron beam, the astigmatism in the quadrupole lens becomes excessive for the side electron beams. In either case, there arises a problem that the resolution in the screen peripheral portions is deteriorated.